A large study on pregnant women in South Korea found that taking antithyroid drugs during the first trimester of pregnancy was associated with a greater risk of congenital malformations.
Pregnancy can often have a profound impact on thyroid function due to changes in hormones. Graves’ disease, an autoimmune disease, is the most common cause of hyperthyroidism during pregnancy. The antithyroid drugs methimazole and propylthioracil had previously been demonstrated to be effective for managing Graves’ disease during pregnancy.
Now, a nationwide study following South Korean women have found associations between prescription for antithyroid drugs and birth defects. The researchers used the National Health Insurance database, which is a prescription claims database for the entire South Korean population, to study 2,886,970 pregnancies with live-born infants between 2008-2010. Of those, 12,891 pregnancies were exposed to antithyroid drugs.
They found that exposure to methimazole and propylthiouracil during the first trimester resulted in relative increases in the risk for congenital malformations of 31% and 16% respectively.
Switching from methimazole and propylthiouracil during the first trimester, or even up to 3 months before pregnancy, did not significantly decrease the overall risk for congenital malformations. Moreover, switching from propylthiouracil to methimazole during pregnancy seemed to increase the absolute risk (by 47 cases per 1000 live births) compared with continuing to receive PTU alone. The researchers think that switching between drugs may pose a substantial risk to the embryo through exposure to different teratogens, rather than eliminating the harmful effects of the prior medication.
These results suggests that, if possible, antithyroid drugs should be avoided in the first trimester of pregnancy. Furthermore, the current recommendation of switching from MMI to PTU after pregnancy detection should be reevaluated in light of this nation-wide research.
Note that a limitation of this study is that it uses a prescription claims database as its assessment of exposure to ATDs. Receiving a prescription did not necessarily mean the person actually used the drug, and the date a prescription was received might not necessarily coincide with when the person actually starts taking the drug.
Link to the study: http://annals.org/aim/article-abstract/2670322/antithyroid-drugs-congenital-malformations-nationwide-korean-cohort-study